Fix Swift 6 actor-isolation warnings in model inits and services

- Message, Conversation, EmailLog: add nonisolated to inits — plain value
  types have no actor isolation, but the macOS 27 SDK was inferring it
- EncryptionService: replace lazy var encryptionKey (which mutates self and
  gets inferred as @MainActor) with an eagerly-initialized let in init()
- FileLogger: add nonisolated to shared, write, and minimumLevel so they
  are callable from nonisolated AppLogger methods without warnings
- LogLevel.<: add nonisolated to the Comparable conformance method

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

oAI/Utilities/Logging.swift

@@ -52,7 +52,7 @@ enum LogLevel: Int, Comparable, CaseIterable, Sendable {
         }
     }
 
-    static func < (lhs: LogLevel, rhs: LogLevel) -> Bool {
+    nonisolated static func < (lhs: LogLevel, rhs: LogLevel) -> Bool {
         lhs.rawValue < rhs.rawValue
     }
 }
@@ -60,7 +60,7 @@ enum LogLevel: Int, Comparable, CaseIterable, Sendable {
 // MARK: - File Logger
 
 final class FileLogger: @unchecked Sendable {
-    static let shared = FileLogger()
+    nonisolated static let shared = FileLogger()
 
     private let fileHandle: FileHandle?
     private let queue = DispatchQueue(label: "com.oai.filelogger")
@@ -70,8 +70,8 @@ final class FileLogger: @unchecked Sendable {
         return f
     }()
 
-    /// Current minimum log level (read from UserDefaults for thread safety)
-    var minimumLevel: LogLevel {
+    /// Current minimum log level (backed by UserDefaults — thread-safe).
+    nonisolated var minimumLevel: LogLevel {
         get {
             let raw = UserDefaults.standard.integer(forKey: "logLevel")
             return LogLevel(rawValue: raw) ?? .info
@@ -95,7 +95,7 @@ final class FileLogger: @unchecked Sendable {
         fileHandle?.seekToEndOfFile()
     }
 
-    func write(_ level: LogLevel, category: String, message: String) {
+    nonisolated func write(_ level: LogLevel, category: String, message: String) {
         guard level >= minimumLevel else { return }
         queue.async { [weak self] in
             guard let self, let fh = self.fileHandle else { return }